1. Field of the Invention
The present invention relates to a device for use in the identification of microorganisms and more specifically to a means for eliminating the problem of false-positive reactions in negative control wells of microorganism identification test containers.
The large number of strains of highly pathogenic bacteria and their varying susceptibility to antibiotics has resulted in the development of techniques for microorganism identification and antibiotic susceptibility testing including the determination of the minimum inhibitory concentration of antibiotic, i.e., the lowest concentration of antibiotic which completely inhibits the growth of the organism. These techniques typically involve the use of test trays which comprise, for example, a one piece plastic shell containing a plurality of receptacles or incubation wells. The wells may be arranged in parallel rows such as described in U.S. Pat. Nos. 3,826,717 and 3,356,462, or in circular patterns such as described in U.S. Pat. No. 4,010,078.
During the antibiotic susceptibility test, the wells contain the microorganisms to be inhibited, nutrient broth and antibiotic. Typically before inoculation each of the antibiotic test wells contains a different concentration of antibiotic and each row of such wells contains a different antibiotic. Each of the wells is inoculated with a standard amount of microorganism. The contents of the wells are incubated for a period of time and are then examined to determine the minimum inhibitory concentration of the appropriate effective antibiotic, U.S. Pat. No. 3,826,717. Those wells which contain a sufficient concentration of a growth inhibiting antibiotic contain a clear solution, while those wells which contain an insufficient concentration of antibiotic or an ineffective growth inhibiting antibiotic have either a turbid solution or a small turbid area or dot surrounded by a clear solution, indicating growth of the microorganisms. Cross-contamination between test wells on the same plate is minimized by the use of barrier elements between rows of adjacent wells and by overlaying the openings of the wells with a cover or other sealing means.
During the microorganism identification test the wells contain the microorganism to be identified, nutrient broth and a biochemical indicator which changes in color in response to the appropriate microorganism, U.S. Pat. No. 4,010,078.
U.S. Pat. No. 3,107,204 discloses the use of an antibiotic test tray having wells containing fibrous discs. The discs contain dehydrated bacteriological media, a dye color indicator in accordance with bacterial growth or activity and various chemotherapeutic agents. The discs are arranged in pairs, each pair containing a high and a low concentration of the same antibacterial agent. Color changes are compared against a control to measure antibiotic activity, but not for identification of any particular active microorganism. The problem of cross-contamination of the materials in adjacent wells is recognized and overcome by the use of a sealing lid.
U.S. Pat. No. 4,010,078 provides a device for use in the identification of microorganisms comprising an open-topped multi-compartmented microorganism culture media receiving portion and a cover member. Several of the types of media employed contain an indicator which changes color in response to the growth of a particular microorganism. The use of a standard or negative control well is not discussed.
Heretofore the prior art has not provided an inexpensive microbial test tray which gives rapid results and not only provides antibiotic sensitivity information but further provides an accurate and reproducible means of microorganism identification through comparisons between negative control wells and biochemical test wells. An additional problem presented by microorganism identification is that many of the known biochemical tests involve only subtle color changes in the test media in response to microorganism growth. Determinations that these subtle color changes have occurred are difficult to make in a reliable manner. It is desired to improve the reliability of microorganism identifications based on subtle color changes in the biochemical test media.